In an electrophotographic copying apparatus, a rotatable photoreceptor drum is generally used as an electrostatic image carrier, and the circumferential surface of the drum is uniformly charged before exposure. In the case where the region of an exposed image corresponding to a document image is smaller than the charged region of the circumferential surface of the photoreceptor drum, or when there is a stain on the document cover, or if copying is conducted with an open document cover, a charge remains on a non-image portion around the document image, toner adheres to the remaining charge, and thereby a stain occurs on a copied paper. Therefore, in a recent conventional copying apparatus, a discharging device is provided around the photoreceptor drum, and charge on the unnecessary portion is discharged before an electrostatic latent image is developed so that toner can not adhere to the unnecessary portion. A light emitting diode array (LED array) is used for the discharging device.
The LED array is set in a housing, and mounted on a substrate having a driving circuit and a printed circuit thereon, and arranged so that the longitudinal direction of the LED array coincides with the direction of the shaft of the photoreceptor drum. As shown in FIG. 8, the LED array is arranged so that the longitudinal direction of the LED array (a discharging device) 2 is in parallel with the direction of the shaft of a photoreceptor drum 1 between a charger 3 placed around the circumferential surface of the photoreceptor drum 1, and an image exposure portion 4. In an image process of the electrophotographic copying apparatus, the photoreceptor drum 1 is rotated clockwise as shown in the drawings, and the charger 3 charges uniformly the circumferential surface of the photoreceptor drum at a predetermined electric potential. Next, a discharger 2 located downstream turns on the LED (a light emitting diode) corresponding to the unnecessary portion of the LED array which is found in accordance with the size of a document to be copied, by CPU control. The above-described operation is conducted in synchronization with the movement of the circumferential surface of the photoreceptor drum 1, and black frame erasing is performed by discharging the unnecessary portion in accordance with the size of the document are performed, so that a latent image of the necessary portion can be formed. Then, the latent image is visualized into a toner image by a developing unit 5. The toner image is transferred onto a transfer sheet by a transfer unit 6, separated from the photoreceptor drum 1 by a separator unit 7, and a copy is obtained. Residual toner is cleaned by a cleaning unit 8, preparing for the next copying cycle. As described above, the LED array is used for discharging and black frame erasing.
Since a document, regardless of its size, is always projected symmetrically to a central line of the photoreceptor drum 1, the discharging operation for black frame erasing is performed on both sides of the photoreceptor drum by the same dimension. An LED driving circuit is based on the circuit in which an LED Li, and a current limitation resistor Ri are connected in series with a power source (for example, D.C 8 volts) and grounded. In practice, there is also a system in which a pair of two LEDs having similar light emitting characteristics, which are connected in series, are driven by a current limitation resistor Ri. In any system, turning-on or turning-off of the LED array is performed by an exclusive driving IC. After a data for turning-on or turning-off is written in a latch memory in the driving IC, and when a driving signal is inputted into the memory, the whole necessary portions of the LED array are turned on at one time. On the other hand, when a signal for turning-off is inputted, the whole LEDs, which are turned on, are turned off at one time. In this driving method, when the LED is turned on, a transient current (a dash-current) is given at one time, an electrolytic capacitor for the power source can not absorb power source variation, and driving voltage of the LED rises up from 8 volts to about 10 volts, resulting in a harmful influence on LED life. Furthermore, when the LEDs are turned off at one time, the driving voltage does not reach the ground level at once, so that residual voltage of .+-.1 to .+-.2 volts remains, and therefore, some LEDs of the LED array which are turned on remain, resulting in a stripe being generated on an image, which are problems. Related to this, since a spike-noise is generated in a driving wave-form of the driving IC during the period from turning-on of the LED to turning-off, and noises and a false operation are generated in each portion in a copying apparatus, a stain may be generated on the copied image and image quality may be lowered.